title: Characterization of a tricationic trigonal bipyramidal iron(IV) cyanide complex, with a very high reduction potential, and its iron(II) and iron(III) congeners.
abstract: Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe(IV)(O)(TMG(3)tren)](2+) (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG(3)tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe(IV)(CN)(TMG(3)tren)](3+) (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe(II)(CN)(TMG(3)tren)](+) (2), via the S = 5/2 complex [Fe(III)(CN)(TMG(3)tren)](2+) (3), the progress of which was conveniently monitored by using UV-vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mössbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe(IV/III) reduction potential of ∼1.4 V vs Fc(+/o), the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t(1/2) in CD(3)CN solution containing 0.1 M KPF(6) at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to (13)C NMR at -40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG(3)tren ligand to support highly charged high-valent complexes.
authors.affiliation: Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Jason
authors.initials: J
authors.lastname: England
authors.forename: Erik R
authors.initials: ER
authors.lastname: Farquhar
authors.forename: Yisong
authors.initials: Y
authors.lastname: Guo
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Cranswick
authors.forename: Kallol
authors.initials: K
authors.lastname: Ray
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
mesh_terms: D002412:Cations / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D002412:Cations; D009942:Organometallic Compounds; D007501:Iron